Re: Stupidity or sequences?
- From: Dba DBA <oracledbaquestions@xxxxxxxxx>
- To: ORACLE-L <oracle-l@xxxxxxxxxxxxx>
- Date: Mon, 22 Apr 2013 16:29:45 -0400
does anyone know what oracle does under the covers to guarantee uniqueness
of the sequence? When you cache sequence values in memory (say 500) per
node... I am guessing they use a struct to store the current value. In
java/c there is a way to make a method/class/struct serialized so only 1
session at a time can get the value. I doubt its anything fancy.
I am guessing the basic algorithm for a sequence with cache 500
Pseudo code below
select sequence_value
from sequence_table_in_data_dictionary
where sequencename = ....
update sequence_table_in_data_dictionary
set sequence_value = old_value+500
where sequence_name = ....
commit;
Then in memory, each sequence gets its own C Struct (below SQL layer)
struct SequenceName serial -- don't remember the C syntax for a Struct or
to serialize a struct
{
current_value number;
max_value number --when this hits, find the new value and increment the
sequence value in the DB\
}
Then getter function for CURRVAL and setter function for NEXTVAL
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